This application is related to U.S. Patent Application Ser. No. 603,704 entitled "In Situ Recovery of Shale Oil", filed Aug. 11, 1975 by Gordon B. French, now U.S. Pat. No. 4,043,595, to U.S. Pat. Application Ser. No. 603,705 entitled "Forming Shale Oil Recovery Retort Into Slot-Shaped Columnar Void", filed Aug. 11, 1975, by Richard D. Ridley, now U.S. Pat. No. 4,043,596, and to U.S. Patent Application Ser. No. 790,350 entitled "In Situ Oil Shale Retort With a Horizontal Sill Pillar" filed Apr. 25, 1977, by Ned M. Hutchins. All three of these applications are assigned to the assignee of the present application and are incorporated herein by this reference.
This invention relates to recovery of liquid and gaseous products from oil shale. The term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen" which upon heating decomposes to produce hydrocarbon liquid and gaseous products. The formation containing kerogen is called "oil shale" herein, and the hydrocarbon liquid product is called "shale oil".
One technique for recovering shale oil includes forming an in situ oil shale retort in a subterranean formation containing oil shale. At least a portion of the formation within the boundaries of the in situ oil shale retort is explosively expanded to form a fragmented permeable mass of particles containing oil shale. The fragmented mass is ignited near the top of the retort to establish a combustion zone. An oxygen-containing gas is introduced into the top of the retort to sustain the combustion zone and cause it to move downwardly through the fragmented permeable mass of particles in the retort. As burning proceeds, the heat of combustion is transferred to the fragmented mass of particles below the combustion zone to release shale oil and gaseous products therefrom in a retorting and vaporization zone. Vaporized constituuents of shale oil, water vapor and the like may condense on cooler oil shale in the retort below the retorting zone. The retorting zone moves from top to bottom of the retort ahead of the combustion zone, and the resulting shale oil and gaseous products pass to the bottom of the retort for collection and removal. Recovery of liquid and gaseous products from oil shale deposits is described in greater detail in U.S. Pat. No. 3,661,423, to Donald E. Garrett, assigned to the assignee of this application.
In preparing for the retorting process the formation containing oil shale should be fragmented rather than simply fractured to create good and uniform permeability so that undue pressures are not required to pass the gas through the retort, and so that valuable deposits of oil shale are not bypassed owing to non-uniform permeability. The aforementioned patent applications disclose techniques for fragmenting a substantial volume of formation in a retort site to form a fragmented mass of particles in an in situ oil shale retort. The in situ retort is formed by excavating a void in the retort site, drilling blasting holes into the remaining portion of the formation in the retort site, loading explosive into the blasting holes, and detonating the explosive to expand the formation toward the void.
To promote maximum uniformity of particle size and permeability of the fragmented mass, and to minimize the quantity of explosives, the blasting holes should be reasonably accurately located with respect to each other, and with respect to the void toward which expansion occurs during the explosion. Oil shale formations in the western United States are often between 50 to about 500 feet thick or even more, and are covered by a non-productive overburden, which may be thousands of feet deep, thus often making it difficult to drill from the surface and accurately locate blasting holes in the oil shale formation.
In one embodiment disclosed in application Ser. No. 790,350, entitled "In Situ Oil Shale Retort With a Horizontal Sill Pillar", an open base of operation is excavated in the new formation at a working level near the top of an in situ retort to be formed, which may be a thousand feet, or more, below the ground surface. A substantially horizontal access drift is excavated at a production level below the base of operation to provide access to a lower portion of the retort site. A void is excavated above the access drift so the void opens into the access drift and terminates below the base of operation at the top of the fragmented mass being formed. This leaves a substantially horizontal portion of unfragmented formation between the top of the void and the bottom of the base of operation. Blasting holes for explosive for expanding formation are drilled from the base of operation into a portion of the formation within the boundaries of the retort being formed. Inasmuch as the working level is much closer to the top of the retort being formed than the distance from the retort through the overburden to the ground surface, this permits more accurate and rapid drilling of blasting holes from the base of operation than from the ground surface. This, in turn, facilitates explosive expansion to form the fragmented mass of oil shale particles in the retort. Explosive is loaded into such blasting holes and detonated for explosively expanding formation towards such a void for forming a fragmented permeable mass of particles containing oil shale in the retort.
In an embodiment disclosed in Application Ser. No. 790,350 entitled "In Situ Oil Shale Retort With a Horizontal Sill Pillar", a horizontal sill pillar of unfragmented formation remains between the top of the fragmented mass in the retort and the bottom of the base of operation. The sill pillar has a number of bore holes through it after formation of the fragmented mass. Such bore holes include the upper ends of blasting holes drilled from the base of operation. Such bore holes can be used for access from the base of operation for establishing and sustaining a combustion zone in the fragmented mass below the sill pillar.
U.S. Pat. No. 3,661,423 to Garrett discloses an in situ oil shale retort in which communication is established with the top of an expanded oil shale deposit by drilling a plurality of communicating conduits to the top of the expanded shale. A source of oxygen from a compressor is then provided to the conduits. To establish a flow of oil from the shale, the upper level of the expanded shale deposit is ignited using an initial supply of fuel and air to the top of the shale deposit through the conduits. A source of oxygen is supplied at a pressure sufficient to overcome the inherent pressure drop through the conduits and the shale deposit to establish a positive downward flow of hot gases.
U.S. Patent Application Ser. No. 716,583, entitled "Method For In Situ Recovery of Liquid and Gaseous Products From Oil Shale Deposits", filed on Aug. 23, 1976, by Gordon B. French, and assigned to the assignee of this application, discloses an in situ oil shale retort in which a plurality of air supply holes, or passages, are drilled from a tunnel to distributed locations at the top retort. One of the air supply holes can extend directly down from the tunnel to the center of the top portion of the fragmented mass in the retort. The other air inlet holes slope from the overlying tunnel to the top portion of the retort near the corners. The air supply holes have diameters of 4 to 7 feet for minimizing pressure losses. The effective sizes of the holes overlying paths through the retort can be selectively changed as by adjusting louvers within such holes.
U.S. Pat. No. 2,481,051 to Uren discloses a plurality of vertical pipes installed through broken shale extending from a lower level to an upper level in an in situ oil shale retort. At the upper end of each pipe a lateral connection is provided for the introduction of compressed air. Compressed air is introduced through an inlet line and then through the lateral connections to the upper end of each of the pipes.
It can be desirable to use the base of operation as a working level from which to control formation of a fragmented mass in an in situ oil shale retort and from which to subsequently regulate the flow of gas through the fragmented mass. By providing separate gas flow control valves connected to bore holes extending through a sill pillar, the relative amounts of gas supplied to selected regions of the fragmented mass can be controlled.
Such control can inhibit non-uniform advancement of a combustion zone through the fragmented mass. By providing a relatively uniform advancement of the combustion zone, more effective retorting of the fragmented mass can be provided, which can result in a greater yield from the retort or more economical operation.